This invention relates generally to modular flat panel display devices, and particularly to a system for correcting for intensity variations and electron beam landing errors in such devices.
A modular flat panel display device, in which the instant invention can be utilized, is described in U.S. Pat. No. 4,117,368 issued to Marlowe, et al. The Marlowe et al. device consists of an evacuated envelope which is divided into channels by a plurality of insulating vanes. Each of the channels includes guide meshes for propagating electron beams along the lengths of the channels. When a particular line of the visual display is to be produced, the electron beams are ejected from the guide meshes and travel toward the display screen. The vanes support deflection electrodes which are biased with varying deflection potentials. These deflection potentials cause the electrons traveling from the guide meshes to the display screen to be scanned transversely across the channels. The electron beams of all the channels are simultaneously ejected from between the guide meshes so that a portion of the same horizontal line of the visual display is simultaneously generated across each of the channels.
Because the visual display of a modular display device is generated across a plurality of channels, the brightness of each channel must be uniform within the perception capability of the human eye. U.S. Pat. No. 4,126,814 issued to Marlowe discloses a system for uniformly controlling the visual display brightness of a modular flat panel display device or the type described in U.S. Pat. No. 4,117,368. The display brightness of the modules must be uniform within approximately 1% to assure that the seams between the modules are invisible. In a modular display device every channel uses a different electron gun. The electron guns and their associated circuitry typically do not have uniform transfer characteristics and, therefore, the display brightness of the channels will vary along with the transfer characteristics variations. The system described in the Marlowe U.S. Pat. No. 4,126,814 addresses this problem by storing the brightness drive voltages required to produce 64 different predetermined levels of picture brightness for each of the electron guns. The stored drive voltages are selected from a voltage range which is divided into 256 equal incremental voltages. The electron gun is driven by the incremental voltages and the voltages produced on an electron collector are compared with a reference voltage. The incremental voltage which brings the collector voltage up to the reference voltage is then stored. This procedure is repeated for 64 reference voltages so that 64 to 256 incremental drive voltages are stored. Accordingly, uniform brightness for all modules in the display is achieved because the brightness levels for all the modules are referenced to the same 64 reference voltages.
The brightness uniformity control system described in the Marlowe U.S. Pat. No. 4,126,814 operates satisfactorily for the purposes intended. However, the electron beams propagate in an oscillatory manner between the guide meshes. This inherently results in wobble of the electron beams during the travel from the guide meshes to the screen. This wobble introduces permanent deleterious display characteristics which are unique to each display device, and to each module within the display device. The first such characteristic caused by electron beam wobble is a sinusoidal variation of the display brightness in the direction of the electron beam propagation between the beam guides. The second deleterious display characteristics induced by electron beam wobble is that of causing the electron beams to land at improper positions on the screen. This characteristic can also be caused by permanent structural features of the display device. Because the features are permanent, the correction for the landing errors they induce also are permanent. One segment of every horizontal line across the display is generated by each of the channels so that each channel contributes a portion of each horizontal line. Electron beam landing errors can cause the line segments of adjacent channels to be slightly displaced thereby disrupting the continuity of the horizontal line.
The instant invention overcomes these deficiencies by the provision of compensation circuitry for overcoming the deleterious display characteristics introduced by electron beam wobble and by structural features of the display device.